Nuclear reactors are classified depending on an installation position of main equipment (steam generator, pressurizer, pump, etc.) into a separate type reactor (e.g., commercial reactor: Korea) in which the main equipment is installed outside a reactor vessel, and an integral type reactor (e.g., SMART reactor: Korea) in which the main equipment is installed inside the reactor vessel.
Unlike general industrial boilers, a nuclear facility (nuclear plant) generates residual heat from a reactor core for a considerable period of time even after the reactor core is shut down by control rods. Accordingly, various safety facilities are intensively installed in the nuclear facility in order to maintain the reactor in a stable state by removing the residual heat of the reactor core when an accident occurs. In addition, various facilities, such as a secondary system, a shutdown cooling system, and the like, which operate during a normal reactor shutdown operation.
A steam generator of a commercial separate type reactor is separated from a reactor and installed at a higher position than the reactor. In a commercial separate type nuclear facility, during a normal reactor cooling operation, a reactor coolant system is primarily cooled using a secondary system, and thereafter a primary system is cooled by directly injecting cooling water using a shutdown cooling system. And, in the commercial separate type nuclear facility, during a normal cooling operation or a cooling operation due to an accident, the reactor coolant system is primarily cooled using a main or auxiliary feed water system and the secondary system, and thereafter the primary system is cooled by directly injecting cooling water using the shutdown cooling system constructing a part of a safety system.
A steam generator of an integral type reactor such as SMART is installed inside a reactor vessel and installed at a position which is higher than a position of a reactor core but lower than the position of the steam generator of the commercial separate-type nuclear reactor. In the integral type reactor having a passive residual heat removal system, during a normal reactor cooling operation, a reactor coolant system is primarily cooled using a feed water system and a secondary system, and thereafter a primary system is cooled by directly injecting cooling water using a shutdown cooling system. And, in the integral type reactor, during a cooling operation due to an accident, the reactor coolant system is primarily cooled using the passive residual heat removal system and the steam generator, and thereafter the primary system is cooled by directly injecting cooling water using the shutdown cooling system.
However, the related art shutdown cooling system having such structure is provided with a line (or a pipe) that is directly connected to the reactor vessel. Accordingly, when the line is damaged, a loss-of-coolant accident may occur. Also, the reactor vessel is opened during the shutdown cooling operation in the related art shutdown cooling system. Thus, in order to secure a suction head of a shutdown cooling pump, the shutdown cooling pump is generally installed at the lowermost part of a nuclear facility containment building and a line of a suction part thereof is designed to be great.
For the integral type reactor, a diameter of a connection nozzle for connecting the line to the reactor vessel is related to strength of a line break accident. Hence, when the diameter of the nozzle increases, an effect of eliminating a large loss-of-coolant accident, which is an inherent characteristic of the integral type reactor, is impacted. Thus, in general, the integral type reactor meets suction head conditions by reducing the diameter of the nozzle connected to the reactor vessel (reactor coolant system) and thereafter increasing the diameter of the connection line. However, in this integral type reactor, flow resistance occupied by the connection nozzle of the reactor vessel is very large, so there is a limit to reduce the flow resistance. Accordingly, a design of gradually cooling the reactor coolant system by adopting a method of reducing a flow rate in the shutdown cooling system is applied. Because of this design characteristic, the integral type reactor generally is required more time to perform the shutdown cooling operation (reaching reload temperature) than the commercial reactor.
Also, in general, the related art shutdown cooling system is directly connected to the reactor coolant system. The shutdown cooling system is thereafter connected to a component cooling system, and the component cooling system is then connected to a seawater system. The reason why the component cooling system is installed between the shutdown cooling system and the seawater system is to prevent cooling water of the reactor coolant system, which is directly connected to the nuclear reactor core and thus contains a radioactive material, from directly exchanging heat with seawater. In other words, an intermediate circulation channel (loop) (component cooling system) is provided to prevent the cooling water of the reactor coolant system from being discharged directly to the seawater when the heat exchanger is damaged. With this configuration, the related art shutdown cooling system requires many related systems and heat exchangers.